Lithium has different embedding energies when the intercalation reaction occurs between the two electrodes, and in order to obtain the performance of the battery, the capacity ratio of the two host electrodes should be kept at an equilibrium value. In a lithium ion battery, the capacity balance is expressed as the mass ratio of the positive electrode to the negative electrode.
Namely: γ=m+/m-=ΔxC-/ΔyC+
In the formula, C refers to the theoretical Coulomb capacity of the electrode, and Δx and Δy refer to the stoichiometric number of lithium ions embedded in the negative electrode and the positive electrode, respectively.
It can be seen from the above equation that the mass ratio required for the two poles depends on the corresponding Coulomb capacity of the two poles and the number of their respective reversible lithium ions. In general, a smaller mass ratio results in incomplete utilization of the negative electrode material; a larger mass ratio may present a safety hazard due to overcharge of the negative electrode. In short, at the optimum quality ratio, battery performance**.
The above is just an ideal lithium-ion battery system. The balance of content does not change during the cycle, but the actual situation is much more complicated. Any side reaction that produces or consumes lithium ions or electrons can cause a change in battery capacity balance. Once the battery's capacity balance changes, the change is irreversible and can be accumulated over multiple cycles to produce battery performance. Serious impact.
In the lithium ion battery, in addition to the redox reaction occurring during lithium ion deintercalation, there are a large number of side reactions such as decomposition of the electrolyte, dissolution of the active material, deposition of lithium metal, and the like.
What are the possible causes of capacity decay?
1. Self-discharge of the battery
Self-discharge refers to the phenomenon that the battery naturally loses its capacity when it is not in use. Lithium-ion battery self-discharge leads to capacity loss in two cases: one is reversible capacity loss; the other is irreversible capacity loss. Reversible capacity loss means that the lost capacity can be recovered during charging, while the irreversible capacity loss is reversed. The positive and negative electrodes may react with the electrolyte in the charged state (microbattery reaction), lithium ion insertion and deintercalation, positive and negative The lithium ions embedded and deintercalated are only related to the lithium ion of the electrolyte, and the positive and negative electrodes are therefore unbalanced, and this capacity loss cannot be recovered during charging.
Such as the reaction of the positive electrode and the solvent: LiyMn2O4+xLi++xe→Liy+xMn2O4
The solvent molecule (such as PC) is oxidized as a microbattery on the surface of the conductive material carbon black or current collector: xPC→xPC-free radical+xe;
Reduction reaction of negative electrode and solute LiPF6
PF5+xe→PF5-x
Lithium carbide in a charged state is oxidized by removing lithium ions as a negative electrode of the microbattery:
LiyC6→Liy-xC6+xLi++xe
2. Electrode instability
Including the structural changes of the electrode during charge/discharge/cycle, and the dissolution of the positive electrode due to structural defects
3. Collector corrosion
Copper and aluminum are the most commonly used materials for the negative and positive current collectors, respectively.
The fluid collector corrosion is related to the electrolyte. In the LiPF6-EC/DMC electrolyte, the voltage is 4.2V (http://vs.Li/Li+), which can corrode the aluminum foil. The aluminum foil is in the air or in the electrolyte. It is easier to form an oxide film on the surface. At the same time, the overall corrosion and local corrosion of the current collector surface (such as pitting) and poor adhesion cause the electrode reaction resistance to increase, and the internal resistance of the battery increases, resulting in capacity loss and discharge efficiency. . The copper current collector corrodes during use to form a film of insulating corrosion product. As a result, the internal resistance of the battery increases, and the discharge efficiency decreases during the cycle, resulting in capacity loss.
In addition, there are overcharge and overdischarge during use, and the decomposition of the electrolyte will cause loss of lithium ion battery capacity.
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